Paints are presently sold in such a way that the customer selects the desired colour shade, after which paint of the desired shade is manufactured by adding one or more paint pastes, i.e. colour pastes or colorants, into a paint can containing suitable basic colour.
Paint is manufactured in a shading apparatus typically comprising a plurality of pumps, for instance 16, each of which is arranged to pump a given colour paste. The pumps are often of a piston pump type. Dispensing takes place essentially in such a way that a piston pump sucks paste to the inside of it from a paste container, to which the piston pump is connected via a three-pass valve. After the piston pump has been filled, the piston of the pump is moved in the opposite direction, whereby paste flows back to the paste container. When there remains the paste amount to be dispensed into the paint can in the pump, the channel to the paste container is closed by means of the three-pass valve, and instead, the paste is directed to a channel leading to a paint can.
The size of the cans for the paint to be shaded typically varies between 0.25 l and 20 l, and the amount of paste to be dispensed is 0.05 ml to 2 l/paste. This causes the problem that shading of the paint in bigger cans may take several minutes if the productivity of the pump is low. A solution is to increase the volume of the piston pump by increasing the cylinder diameter. This brings about a new problem: the accuracy of small amounts of paste deteriorates.